The present invention relates to improved outlet temperature limit operation of gas turbines and more particularly to combined cycle electric power plants in which gas turbines are operated with improved temperature limit operation.
One of the more critical parameters of gas turbine operation is the cycle temperature, and specifically the turbine inlet temperature. The cycle temperature should be high enough to maximize power and operating efficiency and at the same time it should be low enough to protect the turbine and enhance its life. Thus, a relatively narrow temperature band exists for optimum turbine operation and temperature overshoot during any transient due to any system disturbance, such as a load change, should be minimized. Temperature overshoots can be caused when the turbine is operated with temperature limit control as the primary control or when the turbine is operated with a speed/load control as the primary control and a temperature limit control as a backup control.
To prevent the turbine inlet or cycle temperature from exceeding a predetermined value, it is generally necessary to obtain downstream indications of turbine outlet temperature, i.e. blade path or exhaust temperature or both, and develop one or more temperature limit signals for the fuel control in accordance with the temperature indications and a blade path and/or exhaust temperature reference. In turn, the temperature reference is a variable value dependent on combustor shell pressure so that the turbine inlet temperature is held constant as the turbine pressure and temperature drops vary with varying fuel flows and varying ambient temperatures.
Blade path temperature control generally provides faster response than exhaust temperature control since it is based on temperature measurements made at points closer to the turbine inlet, but stratification often occurs in the blade path flow about the turbine annular space, and blade path temperature indications can differ significantly according to the annular location of the thermocouples. At the outlet flow points at which exhaust temperature indications are generated, outlet flow mixing will normally have dissipated any stratification, but the lower mass-velocity results in significantly slower thermocouple response time as compared to the response time in the blade path region where the mass-velocity is much higher.
Temperature limit control has been used as a primary control during startup and as a primary control during loading. Such control has been implemented with the use of a temperature control signal based on blade path temperature as well as with the use of a temperature control based on exhaust temperature. Further, in certain prior art relaypneumatic controls and in the digital/analog hybrid electropneumatic control disclosed in the aforementioned copending application Ser. No. 319,114, a blade path temperature control and an exhaust path temperature control have been employed to generate separate temperature limit values with the final temperature control limit value being a low selected value of the two.
It has been generally desirable further to improve the smoothness and the responsiveness of gas turbine temperature limit operation to achieve better efficiency and turbine life enhancement, and it has been especially desirable to do so in combined cycle plant operation where plant generation capacity is dependent directly on gas turbine availability and indirectly on gas turbine availability to the extent that other power generation apparatus must be cut back in operating level with a gas turbine shutdown. More particularly, it has been desirable to achieve reduced turbine temperature overshoot and generally better transient response on temperature limit control while permitting flexible, efficient and responsive operation of the turbine.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art nor that the interpretation placed on it is unrebuttable.